If it's an original 318 car, it doesn't matter much about not being numbers matching. To be honest, the numbers matching stuff seem to be more of a matter to big block cars. I don't know why.
In this case, it would depend on how much rust we are dealing with. It has been sitting... but where? A garage, or a field? Given what you have said... UGH, I would need some pictures to go above your 15K price tag. I know we are talking about a convertible, but the rust issue scares me to heck.
Mike